1. Field of the Invention
The present invention relates to a linear solenoid.
2. Description of Related Art
JP-A-2004-144230 describes an electromagnetic hydraulic control valve shown in FIG. 5A, and the control valve includes a spool valve 21 and a linear solenoid 1 to drive the spool valve 21. The linear solenoid 1 has a coil 2, a plunger 7, and a magnetic stator 31. The magnetic stator 31 is a component constituting a magnetic circuit, and includes a stator core 6 and a yoke 8. The plunger 7 is arranged inside of the stator core 6. The yoke 8 is made of magnetic member, and has an approximately cup shape to cover an outer periphery of the coil 2.
The stator core 6 integrally has a magnetism attraction core 3, a magnetism transmittance core 5 and a magnetism blocker 4. The magnetism attraction core 3 attracts the plunger 7 in an axis direction by using magnetism. The magnetism transmittance core 5 has a tube shape to cover the plunger 7, and the plunger 7 directly slides on the core 5. The blocker 4 inhibits magnetic coupling between the attraction core 3 and the transmittance core 5. The plunger 7 is driven in the axis direction by changing current value supplied to the coil 2. The driven plunger 7 displaces a spool 23 of the spool valve 21 in the axis direction.
In the linear solenoid 1 of FIG. 5A, the stator core 6 is inserted into the yoke 8 through an opening of the yoke 8, and a left end of the stator core 6 adjacent to the attraction core 3 is fixed to an edge of the opening of the yoke 8. As shown in FIG. 5B, a right end of the stator core 6 adjacent to the transmittance core 5 is not fixed to a bottom 8a of the cup-shaped yoke 8. The transmittance core 5 has a free edge corresponding to right side in FIG. 5B, and the free edge is located inside of a concave 34 defined in center section of the bottom 8a of the yoke 8. Magnetic flux is transmitted between an inner circumference face of the concave 34 of the yoke 8 and an outer circumference face of the transmittance core 5 in a radial direction.
Sufficient assembling clearance α is necessary between the free edge of the transmittance core 5 and a wall of the concave 34 in the radial direction. The clearance α absorbs a product variation or axial gap error of the stator core 6. However, as the clearance α becomes larger, a density of the magnetic flux is lowered, because a magnetic circuit is constituted through the clearance α. In this case, an attraction performance of the plunger 7 is lowered.
Moreover, the clearance α may have a variation in the radial direction, due to attachment tolerance of the stator core 6, for example. At this time, the magnetic flux easily flows through a narrower clearance concentratedly when electricity is supplied to the coil 2. That is, a bias of the magnetic flux will be generated when the magnetic flux is transmitted between the plunger 7 and the transmittance core 5 in the radial direction. Further, a lateral force may be generated to the plunger 7 in the radial direction by the bias of the magnetic flux. The lateral force is applied in a direction in which the bias of the magnetic flux is generated. In this case, the plunger 7 and the stator core 6 may be prevented from having smooth sliding operation.
JP-A-2006-307984 discloses a linear solenoid 1 shown in FIG. 6A to solve the above disadvantages. The linear solenoid 1 includes a ring core 11P located between a coil accommodation resin 9 and a bottom 8a of a yoke 8 in an axis direction. The coil accommodation resin 9 corresponds to a bobbin having a coil 2, or a secondary molding resin molding the bobbin. The ring core 11P magnetically couples the yoke 8 and a transmittance core 5. The ring core 11P is fitted around an outer circumference of the transmittance core 5. Magnetic flux is transmitted between the ring core 11P and the transmittance core 5 in the radial direction, and is transmitted between the ring core 11P and the bottom 8a of the yoke 8 in the axis direction.
The linear solenoid 1 further includes a biasing portion 13P to press the ring core 11P toward the bottom 8a of the yoke 8 in the axis direction. Thus, the magnetic coupling between the ring core 11P and the bottom 8a of the yoke 8 can be enhanced. The biasing portion 13P is an elastic component, for example, a ring-shaped rubber or spring. The biasing portion 13P is compressed between the coil accommodation resin 9 and the ring core 11P in the axis direction. Therefore, a space for locating the biasing portion 13P is necessary between the resin 9 and the ring core 11P in the axis direction, in a case where the linear solenoid 1 has the ring core 11P. A total dimension of the linear solenoid 1 in the axis direction may become long by the length of the biasing portion 13P.
A dimension of the ring core 11P or the coil 2 in the axis direction may be made short so as to reduce the space of the biasing portion 13P. However, if the dimension of the ring core 11P is shortened in the axis direction, a transmitting amount of magnetic flux is reduced, because opposing area between the ring core 11P and the transmittance core 5 is decreased. In this case, a density of the magnetic flux in the linear solenoid 1 is lowered, and attraction performance of the plunger 7 is lowered. In contrast, if the dimension of the coil 2 is shortened in the axis direction, magnetic force generated by the coil 2 is reduced, and attraction performance of the plunger 7 is lowered.